Use NonNull pointer in the place of references in AtomicRef and AtomicRefMut to avoid noalias related soundness bug.

[Imberflur]

With this test added:

#[test]
fn drop_and_borrow_in_fn_call() {
    fn drop_and_borrow(cell: &AtomicRefCell<Bar>, borrow: AtomicRef<'_, Bar>) {
        drop(borrow);
        *cell.borrow_mut() = Bar::default();
    }

    let a = AtomicRefCell::new(Bar::default());
    let borrow = a.borrow();
    drop_and_borrow(&a, borrow);
}

Running cargo +nightly miri test produces:

test drop_and_borrow_in_fn_call ... error: Undefined Behavior: not granting access to tag <236157> because that would remove [SharedReadOnly for <236092>] which is strongly protected because it is an argument of call 64948
   --> /home/imbris/projects/atomic_refcell/src/lib.rs:144:33
    |
144 |                 value: unsafe { &mut *self.value.get() },
    |                                 ^^^^^^^^^^^^^^^^^^^^^^ not granting access to tag <236157> because that would remove [SharedReadOnly for <236092>] which is strongly protected because it is an argument of call 64948
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
help: <236157> was created by a SharedReadWrite retag at offsets [0x8..0xc]
   --> /home/imbris/projects/atomic_refcell/src/lib.rs:144:39
    |
144 |                 value: unsafe { &mut *self.value.get() },
    |                                       ^^^^^^^^^^^^^^^^
help: <236092> is this argument
   --> tests/basic.rs:96:51
    |
96  |     fn drop_and_borrow(cell: &AtomicRefCell<Bar>, borrow: AtomicRef<'_, Bar>) {
    |                                                   ^^^^^^
    = note: BACKTRACE (of the first span):
    = note: inside `atomic_refcell::AtomicRefCell::<Bar>::borrow_mut` at /home/imbris/projects/atomic_refcell/src/lib.rs:144:33: 144:55
note: inside `drop_and_borrow_in_fn_call::drop_and_borrow`
   --> tests/basic.rs:98:10
    |
98  |         *cell.borrow_mut() = Bar::default();
    |          ^^^^^^^^^^^^^^^^^
note: inside `drop_and_borrow_in_fn_call`
   --> tests/basic.rs:103:5
    |
103 |     drop_and_borrow(&a, borrow);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside closure
   --> tests/basic.rs:95:33
    |
94  | #[test]
    | ------- in this procedural macro expansion
95  | fn drop_and_borrow_in_fn_call() {
    |                                 ^
    = note: this error originates in the attribute macro `test` (in Nightly builds, run with -Z macro-backtrace for more info)

This PR replaces the reference with NonNull<T> which is the same fix applied for std::cell::RefCell.

[Imberflur]

Forgot Send/Sync impls! They are added now.

[cuviper]

I think you'll want to update Debug too, like I had to fix in rust-lang/rust#97225.

[Imberflur]

I updated Debug, thanks for the note!

[bholley]

Thanks. Sorry for the long-delay reviewing this, I had to page a bunch of stuff back in. I've released this as 0.1.10.